Powder metal technology is well known to the persons skilled in the art and generally comprises the formation of metal powders which are compacted and then subjected to an elevated temperature so as to produce a sintered product.
Conventional sintering occurs at a maximum temperature of approximately up to 1,150.degree. C. Historically the upper temperature has been limited to this temperature by sintering equipment availability. Therefore copper and nickel have traditionally been used as alloying additions when sintering has been conducted at conventional temperatures of up to 1,150.degree. C., as their oxides are easily reduced at these temperatures in a generated atmosphere, of relatively high dew point containing CO, CO.sub.2 and H.sub.2. The use of copper and nickel as an alloying material is expensive. Moreover, copper when utilized in combination with carbon as an alloying material and sintered at high temperatures causes dimensional instability and accordingly the use of same in a high temperature sintering process results in a more difficult process to control the dimensional characteristics of the desired product. Manufacturers of metal powders utilized in powder metal technology produce pre-alloyed iron powders which are generally more difficult to compact into complex shapes particularly at higher densities (&gt;7.0 g/cc). Manganese and chromium can be incorporated into pre-alloyed powders provided special manufacturing precautions are taken to minimize the oxygen content, for example, by oil atomization. Notwithstanding this, these powders still have poor compressabilities compared to admixed powders.
Conventional means to increase the strength of powder metal articles use up to 8% nickel, 4% copper and 1.5% molybdenum, in pre-alloyed, partially pre-alloyed, or admixed powders. Furthermore double press double sintering can be used for high performance parts as a means of increasing part density. Conventional elements are expensive and relatively ineffective for generating mechanical properties equivalent to wrought steel products, which commonly use the more effective strengthening alloying elements manganese and chromium.
Moreover, conventional technology as disclosed in U.S. Pat. No. 2,402,120 teach pulverizing material such as mill scale to a very fine sized powder, and thereafter reducing the mill scale powder to iron powder without melting it.
Furthermore, U.S. Pat. No. 2,289,569 relates generally to powder metallurgy and more particularly to a low melting point alloy powder and to the usage of the low melting point alloy powders in the formation of sintered articles.
Yet another process is disclosed in U.S. Pat. No. 2,027,763 which relates to a process of making sintered hard metal and consists essentially of steps connected with the process in the production of hard metal. In particular, U.S. Pat. No. 2,027,763 relates to a process of making sintered hard metal which comprises producing a spray of dry, finely powdered mixture of fusible metals and a readily fusible auxiliary metal under high pressure producing a spray of adhesive agent customary for binding hard metals under high stress, and so directing the sprays that the spray of metallic powder and the spray of adhesive liquid will meet on their way to the molds, or within the latter, whereby the mold will become filled with a compact moist mass of metallic powder and finally completing the hard metallic particle thus formed by sintering. U.S. Pat. No. 4,707,332 teaches a process for manufacturing structural parts from intermetallic phases capable of sintering by means of special additives which serve at the same time as sintering assists and increase the ductility of the finished structural product.
Finally, U.S. Pat. No. 4,464,206 relates to a wrought powder metal process for pre-alloyed powder. In particular, U.S. Pat. No. 4,464,206 teaches a process comprising the steps of communicating substantially non-compatible pre-alloyed metal powders so as to flatten the particles thereof heating the communicated particles of metal powder at an elevated temperature, with the particles adhering and forming a mass during heating, crushing the mass of metal powder, compacting the crushed mass of metal powder, sintering the metal powder and hot working the metal powder into a wrought product.
Moreover, various methods have heretofore been utilized to densify a powder metal article. For example, U.S. Pat. No. 4,059,879 teaches a method for partially densifying a selected surface portion of a sintered pores powder metal element.
Furthermore, U.S. Pat. No. 3,874,049 teaches a method of making a powder metal part having a bearing surface. Finally, U.S. Pat. No. 3,365,770 teaches a method of producing a multi-layer bearing while U.S. Pat. No. 3,183,086 teaches a method of making pores body with imperviously sealed surface.
The processes as described in the prior art above present a relatively less cost effective process to achieve the desired mechanical properties of the sintered product. Furthermore, the method described in the prior art above produce powder metal bearing surfaces which do not have desirable strength or wear resistant characteristics.
It is an object of this invention to provide an improved process for producing powder mutual bearings having improved dynamic strength characteristics and an accurate method to control the manufacture of same.
It is an aspect of this invention to produce a method of producing bearing surfaces from powder metal articles comprising blending carbon and ferro alloys and lubricant with compressible elemental iron powder pressing the blending mixture to form the powder metal article, high temperature sintering the powder metal article in a reducing atmosphere then compressing the powder metal article so as to produce a densified layer having a bearing surface then heat treating the densified layer.
It is another aspect of this invention to provide a method of producing bearings from a compacted and sintered cylindrical article by applying a rolling pressure against the cylindrical blank so as to produce a densified layer defining the bearing and then heat treating the densified layer. In one particular preferred embodiment the compacted and sintered cylindrical blank is comprised of between 0.5 to 2.0% chromium, between 0 to 1.0% molybdenum, and between 0.1 to 0.6% carbon composition with the remainder bearing iron and unavoidable impurities.
It is a further aspect of this invention to provide a powder metal bearing comprising a compacted and sintered article having a compacted surface so as to present a densified layer defining the bearing. In one particular embodiment, the compacted sintered article has a composition of between 0.5 to 2.0% chromium, between 0 to 1.0% molybdenum and between 0.1 to 0.6% carbon composition with the remainder being iron and unavoidable impurities. Furthermore, in one particular embodiment, the powder metal bearing has a densified layer with a thickness of up to 2 millimeters with the density of the layer gradually increasing to approximately 98% at the surface of the bearing.